1. Field of Invention
The present invention is directed generally to methods and apparatus based on optoelectronic effect, electro/dielectro-phoresis, and impedance spectroscopy, in order to trap, move, deform and characterize particles, such as cells, molecules, any type of colloids, any of inorganic and bio-organic substances, beads, as well as pucks and like small things. “Optoelectronic Probe” refers to the invention described herein.
2. Background of the Invention
I. Optical Tweezers
The manipulation of micro- or nano-size particles is considered as the key for the new generation of photonic, optoelectronic, and electronic devices, as well as biochemical analysis systems. Optical tweezers is one of the most unique invention in this area and was first successfully demonstrated by A. Ashkin et al. in pioneering works in 1985. (Ashkin, A.; Dziedzic, J. M., “Observation of Radiation-Pressure Trapping of Particles by Alternating Light Beams”, Phys. Rev. Lett. 54, pp 1245–1248 (1985)) The technique of optical tweezers is based on the forces of radiation pressure. These are dipole- or gradient-forces arising from the momentum of the light itself. To make these forces large enough to accelerate, decelerate, deflect, guide, and even stably trap small particles, one has to use continuous wave coherent laser beams to achieve the high intensities and high intensity gradients. Combined with other techniques, optical tweezers can also be a unique tool to characterize the trapped particle. For example, laser fluorescence techniques give increased opportunities to a proper identification of different types of biological objects or labeling.
Although the Optical Tweezers is a very powerful tool, it also has its limitations, such as: 1) that the trapping zone is rather small (on the order of the light wavelength); and 2) focusing the beam leads to very high intensities that can endanger the integrity of biological objects.
II. Electrophoresis/Dielectrophoresis Based Arts
When it is exposed to an electrical field, a charged particle will experience a force and the resulting motion is called as electrophoresis (EP). A neutral particle can also be polarized under electrical field. If a nonuniform direct current (DC) or alternating current (AC) field exists, the polarized particle will move towards or away from regions of high electric-field intensity. This motion is a result of interaction between the field and dipole moment induced in a particle and is called dielectrophoresis (DEP).
The dielectrophoretic force on the particle varies with the frequency of the applied electric field. At the low frequency, the polarity of the dielectrophoretic force on the particle depends on the conductivity difference between the particles and electrolyte. On the other hand, at the high frequency the polarity of the dielectrophoretic force on the particle depends on the permettivity difference between the particles and electrolyte. If the particle is more conductive than the electrolyte around it, the dipole aligns with the field and the force acts up the field gradient towards the region of highest electric field. This effect is called positive dielectrophoresis (PDEP). If the particle is less polarisable than the electrolyte, the dipole aligns against the field and the particle is repelled from regions of high electric field (Hughes, “AC Electrokinetics: Applications for Nanotechnology”, Nanotechnology, 11, pages 124–132, (2000)). This effect is called negative dielectrophoresis (NDEP).
Recently, both EP and DEP have captured much interest because they are effective ways to trap, move, deform and separate particles ranging from colloidals to DNA strands and biological cells (Huang, Y; Ewalt, K. L; et al; “Electric Manipulation of Bioparticles and Macromolecules on Microfabricated electrodes”, Anal Chem, 73, pp. 1549–1559, (2001)). In most cases, embedded electrodes were carefully designed and fabricated by semiconductor processing techniques on substrates, such as silicon, glass or plastics.
The field-induced assembly method is a unique application of Electrophoresis/Dielectrophoresis technology. The precise assembly of two- and three-dimensional colloidal on Conductive ITO electrode surfaces may be induced by an AC or DC electrical field that is normal to the electrode surfaces (U.S. Pat. Nos. 5,855,753, and 6,033,547). This technology was extended on silicon electrode, on which the formation, placement, and rearrangement of planar colloidal arrays can be effected by an external illumination pattern due to the photo-assisted impedance modulation. According to Seul et al, it is necessary to apply an AC electrical field to penetrate the thin oxide existing on the silicon surface (U.S. Pat. Nos. 6,251,691, 6,387,707, 6,468,811, 6,514,771, 6,706,163, and 6,797,524). A optoelectronic tweezers has also been demonstrated by Chiou et al. (Chiou, P. Y; Chang, Z; et al; Proc. IEEE/LEOS International Conference Optical MEMS, pp. 8–9, (2003)) The impedance of an amorphous silicon layer, covered by a silicon nitride layer, is modulated by a laser beam. The particles inside the electrolyte are polarized by a non-uniform AC field and pushed away from the illuminated region by the negative dielectrophoresis force. Those prior arts show superperformances on particle manipulation, but still have their limitations, such as: 1) inability to characterize particle electrically; 2) lack of advantages related to DC electric field; and 3) that the depletion layer at the semiconductor surface and the polarities switching with the AC signal make it very hard to precisely control the electric field applied on the particles.
Ozkan et al have developed an optical addressing scheme to localize polymer beads on an unpatterned silicon surface based on a DC electric field (Ozkan et al, “Heterogeneous Integration through Electrokinetic Migration”, IEEE Engineering in Medicine and Biology, Nov/Dec, pp144 (2001), or Ozkan et al, “Optical Addressing of Polymer Beads in Microdevices”, Sensor and Materials, Vol 14, No 4, pp189–197, (2002)). This approach utilizes an optical microbeam that is directed on the substrate to create an active ‘virtual’ electrode (U.S. Pat. No. 6,605,453). The localized charge is defined by the characteristics of the silicon-electrolyte interface in the electrochemical system and serves to attract oppositely charged objects within the solution. Without a layer of oxide inserted between the silicon and electrolyte, DC voltage was able to be used to manipulate the particles. This technique also has its limitations, such as: 1) undesired effects of the dark current; 2) that high-voltage biasing during the patterning process must be avoided due to the electrolysis reaction; 3) lack of advantages from the frequency response of particles on AC field; and 4) inability to characterize particle electrically.
In summary, none of the previous efforts in this field disclose all of the benefits of the present invention, nor does the prior art teach or suggest all of the elements of the present invention.
III. Impedance Spectroscopy
Electrical impedance spectroscopy (EIS) is widely used in experimental studies to characterize living cell. For example, EIS can reflect the size, shape, and density of cells in tissue as well as the conductivity of intra and extra cellular milieu. This allows the identification of difference between tissues or between physiopatological states of the same tissue. The typical way to perform EIS on samples of tissue is the frequency sweep, with frequency range from several Hz to several MHz.
Single cell analysis using DEP and micro electrical impedance spectroscopy (u-EIS) was demonstrated on bovine chromaffin cells and red blood cells (Swomitra K, et al, “A Micro System Dielectrophoresis and Electrical Impedance Spectroscopy for Cell Manipulation and Analysis”, TRANSDUCERS'03, pp 1055–1058, (2003)). A micro scale electrophysiological analysis system was fabricated by micromaching technologies and cells were injected into a microreservoir. Either a vacuum or DEP was utilize to move cells in the channel and position them between platinum electrodes for impedance analysis.
IV. MIS and EIS Tunnel Junction
The metal-insulator-semiconductor (MIS) structure has been proved to be extremely useful in semiconductor devices. When an ideal MIS structure is biased with positive or negative voltages, four cases may exist at the semiconductor. They are accumulation, depletion, inversion, and deep depletion cases. (S. M. Sze, “The Physics of Semiconductor Devices”, 2nd edition, Chapt. 7, pp362–370, Wiley Interscience (1981))
Let us use n-type semiconductor as an example. When a positive voltage is applied to the metal plate, the energy bands near the semiconductor surface are bent downward. According to semiconductor theory, the downward bending of the energy bands at the semiconductor surface gives rise to an enhanced concentration, an accumulation of electrons near the insulator-semiconductor interface. This is called the accumulation case.
When a small negative voltage is applied to the metal electrode of an ideal MIS structure, the energy bands bend upward. The majority carriers, electrons here, are pushed away from the surface by the electric field and depleted at the surface. This is called the depletion case. The surface region (layer), in which the majority carriers (electrons) are depleted, is called depletion region (layer). In the depletion case, the depletion layer will shield a significant amount of applied electric field.
According to the semiconductor theory, the hole concentration at the semiconductor surface is in proportion to the degree of the upward band bending. When a larger negative bias is applied, the bands bend upward even more and the hole concentration at the semiconductor surface may be larger than the intrinsic carrier concentration and the electron concentration at the surface becomes less than the intrinsic carrier concentration. The number of holes (minority carriers) at the surface is greater than the number of electrons (majority carriers); the surface is thus inverted. This is called the inversion case and there is an inversion layer at the insulator-semiconductor interface. As the band are bent further, eventually the hole concentration at the surface will be equal to or higher than the original electron concentration in the n-type semiconductor material. Typically, the width of the inversion layer ranges from 1 nm to 10 nm and is always much smaller than the surface depletion layer width. The inversion layer, after it is formed, will shield most of the applied electric field and work as a perfect electrode similar to a piece of metal.
In addition to the bias condition, the minority carrier concentration at the insulator-semiconductor interface also depends on the interaction between the supply capability of minority carriers and the leakage current through the insulator. Under the condition that the minority carrier concentration at the insulator-semiconductor interface is dominated by the leakage process, the minority carriers (holes for n-type semiconductor) will leak through the insulator and therefore the inversion layer can not be formed or maintained at the insulator-semiconductor interface. The semiconductor surface stays in the depletion region and this is called the deep depletion case.
When the thickness of the insulator layer is less than 5 nm, quantum tunneling phenomena plays significant role for a MIS structure. For a MIS tunnel junction formed on lightly doped semiconductor in the reverse bias region, the degree of inversion at semiconductor/ultra-thin insulating layer interface depends on the supply rate of minority carriers to the surface for a MIS tunnel junction (Green, M. A; Shewchun, J; Solid-State Electron, 17, pp. 349–365, (1974)). Under the condition without minority carrier injection, an inversion layer cannot be maintained at the interface due to the fact that minority carriers leak through ultra-thin insulating layer, due to the tunneling process. A significant portion of bias will drop in the depletion region in the semiconductor and the semiconductor is in the deep depletion region. With the help of external minority carries injection, such as illumination, an inversion layer can be built at the interface and will shield electrical field. As we know, the electron occupation can be characterized by an energy level, called the Fermi level, which will change along with applied bias. In the inversion case, bias will primarily drop on that ultra-thin insulating layer. As a result, the Fermi level in the metal electrode will move to majority energy band edge and a majority carrier tunnel current can be triggered. Theoretically, this process can result in the multiplication of any minority carrier current injected to the insulator-semiconductor interface by factors of 100–1000.
A localized multiplication process was also observed by this author in a prior study on a nano-size MIS tunnel junction formed by a STM tip on lightly doped silicon (Lin, Hai-An et al, Appl. Phys. Lett. Vol73 pp. 2462–2464, (1998)). In that case, the minority carriers were injected by illumination too and MIS tunnel diodes can work as a photo switch. It has been demonstrated that current multiplication can occur in a suitably biased MIS tunnel diode.
The characterization of electrolyte-insulator-semiconductor (EIS) junction is very similar to that MIS junction, except the metal in the MIS junction is replaced by an electrolyte in the EIS junction. One of well known EIS structures is the ion-sensitive-filed-transistor (ISFET). The ISFET is constructed by substituting a sensing film for the metal gate on the gate oxide of a traditional MOSFET and using electrolyte to apply the gate voltage. When the thickness of the insulator in an EIS junction is less than 5 nm, an EIS tunnel junction will be formed.